EU citizens need to apply to stay in the UK, but community organisations warn vulnerable groups may be at risk.
And if just 5% of applicants have their bid to remain in the UK refused or disrupted by problems, 175,000 people could be left without legal status.
In a trial conducted with the government pilot, 65 members of the Roma Support Group applied for settled status using the app. Mr Bica told Sky News only seven could complete the application unaided.
"People are really really really concerned about this," he said. "But our capacity to help them is limited.
"Last year we worked with 3000 individuals, out of perhaps 200,0000 in the whole Roma community. We believe the majority have no idea what settled status is."
The government has insisted it has learned the lessons of the Windrush scandal and will support applicants to show their residency using a wide range of different documents.
Among the measures to be taken are translation, a phone line for people experiencing problems, and even providing support to people in their homes if they have limited mobility.
European embassies are also offering support to their citizens, running advice clinics for people worried about their status.